1. Field of the Invention
This invention generally relates to magnetic materials and, more particularly, to rare earth-containing powders, compacts and permanent magnets, and a process for producing the same.
2. Description of the Prior Art
Permanent magnet materials currently in use include alnico, hard ferrite and rare earth/cobalt magnets. Recently, new magnetic materials have been introduced containing iron, various rare earth elements and boron. Such magnets have been prepared from melt quenched ribbons and also by the powder metallurgy technique of compacting and sintering, which was previously employed to produce samarium cobalt magnets.
Suggestions of the prior art for rare earth permanent magnets and processes for producing the same include: U.S. Pat. No. 4,597,938, Matsuura et al., which discloses a process for producing permanent magnet materials of the Fe--B--R type by: preparing a metallic powder having a mean particle size of 0.3-80 microns and a composition consisting essentially of, in atomic percent, 8-30% R representing at least one of the rare earth elements inclusive of Y, 2 to 28% B and the balance Fe; compacting; and sintering the resultant body at a temperature of 900.degree.-1200.degree. C. in a reducing or non-oxidizing atmosphere. Co up to 50 atomic percent may be present. Additional elements M (Ti, Ni, Bi, V, Nb, Ta, Cr, Mo, W, Mn, Al, Sb, Ge, Sn, Zr, Hf) may be present. The process is applicable for anisotropic and isotropic magnet materials. Additionally, U.S. Pat. No. 4,684,406, Matsuura et al., discloses a certain sintered permanent magnet material of the Fe--B--R type, which is prepared by the aforesaid process.
Also, U.S. Pat. No. 4,601,875, Yamamoto et al., teaches permanent magnet materials of the Fe--B--R type produced by: preparing a metallic powder having a mean particle size of 0.3-80 microns and a composition of, in atomic percent, 8-30% R representing at least one of the rare earth elements inclusive of Y, 2-28% B and the balance Fe; compacting; sintering at a temperature of 900.degree.-1200.degree. C.; and, thereafter, subjecting the sintered bodies to heat treatment at a temperature lying between the sintering temperature and 350.degree. C. Co and additional elements M (Ti, Ni, Bi, V, Nb, Ta, Cr, Mo, W, Mn, Al, Sb, Ge, Sn, Zr, Hf) may be present. Furthermore, U.S. Pat. No. 4,802,931, Croat, discloses an alloy with hard magnetic properties having the basic formula RE.sub.1-x (TM.sub.1-y B.sub.y).sub.x. In this formula, RE represents one or more rare earth elements including scandium and yttrium in Group IIIA of the periodic table and the elements from atomic number 57 (lanthanum) through 71 (lutetium). TM in this formula represents a transition metal taken from the group consisting of iron or iron mixed with cobalt, or iron and small amounts of other metals such as nickel, chromium or manganese.
However, prior art attempts to manufacture permanent magnets utilizing powder metallurgy technology have suffered from substantial shortcomings. For example, crushing is typically carried out in a crushing apparatus using an organic liquid in a gas environment. This liquid may be, for example, hexane, petroleum ether, glycerin, methanol, toluene, or other suitable liquid. A special liquid environment is utilized since the powder produced during crushing is rare earth metal based and, accordingly, the powder is chemically active, pyrophoric and readily oxidizable. However, the aforementioned liquids are relatively costly and pose a potential health hazard due to their toxicity and flammability. Furthermore, crushing an alloy mass to make suitable powder in the aforementioned environment is also disadvantageous since the powder produced has a high density of certain defects in the crystal structure which adversely affect the magnetic properties. Additionally, crushing in the organic liquid environment unduly complicates the attainment of the desired shape, size, structure, magnetic field orientation and magnetic properties of the powders and resultant magnets since the organic liquid environments have a relatively high viscosity which interferes with achieving the desired results. Moreover, attempts to passivate the surfaces of the powder particles by coating them with a protective substance, such as a resin, nickel or the like, during and after crushing is a generally ineffective and complicated process which increases the cost of manufacturing.